The present invention relates to a method of cleaning articles and more particularly to a method of cleaning articles using water and organic solvents.
In many industrial processes, it is necessary to clean articles, either during finishing or before further processing in order to remove contaminants such as grease, salts and other water-soluble and water-insoluble impurities. Examples of articles which need to be cleaned include those with glass, metal or polymeric surfaces, such as printed circuit boards.
Articles of this type are typically cleaned using either an organic solvent formulation or an aqueous formulation. Organic solvent formulations are particularly suitable for removing non-polar contaminants, such as hydrocarbons. A range of different organic solvents are typically utilised, such as the isoparaffinic solvents disclosed in EP 1338680 and the glycol ethers disclosed in U.S. Pat. No. 5,876,510. Chlorinated and fluorinated hydrocarbons have also been used as a cleaning agent in, for example, WO 0236531, U.S. Pat. No. 3,424,805, and EP 1042257. However, for environmental reasons, the use of halogenated liquids is now considered less desirable.
It is a current preferred method of cleaning articles to use flammable hydrocarbons at a temperature above their flash point. Such a technique is disclosed in U.S. Pat. No. 5,690,751.
Aqueous formulations are particularly suitable for removing polar contaminants, such as metal salts. The salts dissolve in the aqueous formulation. It is conventional when utilising aqueous formulations to incorporate various other components in addition to water, particularly surfactants, acidic or basic compounds, and corrosion inhibitors, such as the formulations described in U.S. Pat. No. 5,114,607.
It is also known to use emulsions of water and organic solvent. DE 1298836 describes an emulsion comprising chlorinated hydrocarbons and water. DE 102005011236 describes a method of cleaning using the above emulsion in a closed loop machine. EP 1191095 describes a method of cleaning using an azeotropic solvent-water mixture having a miscibility gap.
It is known, in order to combine the effects of both organic and aqueous solvents, to employ both types of solvent sequentially. U.S. Pat. No. 5,876,510 discloses the use of a first cleaning agent which is a mixture of organic solvent and water, with the organic solvent in excess, followed by rinsing with a mixture in which the water is in excess.
EP 1338680 discloses a method of cleaning parts in which a mixture of isoparaffinic solvents is applied in a first washing step, followed by a second washing step using an aqueous solution containing detergents and finally followed by a rinsing step using deionized water. This method shows good cleaning properties, but requires a large amount of water to clean the parts, including the subsequent rinsing step. In addition, due to the time taken to fill the cleaning chamber with water and then empty it, as well as the time taken to dry water-wet parts, there is a long cycle time compared with the methods which use only one solvent.
Furthermore, this method is also relatively energy inefficient.
U.S. Pat. No. 5,120,370 discloses a cleaning process in which the article is cleaned using superheated steam. In some embodiments, a solvent can be present in the chamber at the same point as the superheated steam. The constant flow of superheated steam helps remove the dust. However, neither steam nor solvent will condense on the surface of the article for sufficient time to dissolve the polar and non-polar contaminants. The solvent does not remove water from the surface of the article. In addition, superheated steam is more expensive to produce than non-superheated steam due to the additional energy requirements.
Accordingly, there is a requirement for a method of cleaning parts which provides excellent cleaning, but with reduced cycle times. In addition, it would be beneficial if such a method had a low wastage of solvent and water and good energy efficiency.